OPSMN-4422: Fixed the dropdown widget (os-select) to set the cleared field value as null (instead of undefined) so that AngularJS serialises the field value in the resultant JSON sent to the APIs.
OPSMN-4422: Fixed the dropdown widget (os-select) to set the cleared field value
as null (instead of undefined) so that AngularJS serialises the field value in
the resultant JSON sent to the APIs.

krishagni/openspecimen,krishagni/openspecimen,krishagni/openspecimen